1. Field of the Invention
The invention relates to an ink-jet system which includes an ink channel disposed between an ink reservoir and a nozzle. A pressurizing device is arranged adjacent to the ink channel for generating in the ink liquid an acoustic pressure wave propagating in the ink channel, so that an ink droplet is expelled from the nozzle.
2. Description of Background Art
Ink-jet systems are used as printheads in ink-jet printers. A drop-on demand ink-jet system of the type indicated above is disclosed, for example, in EP-A1-0 402-172. In this system, the ink channel is formed in a substrate which is sandwiched between a bottom plate and a cover plate such that the top and bottom surfaces of the ink channel are formed by the cover plate and the bottom plate, respectively. The ink channel has a constant depth which is identical to the height of the nozzle, but has a larger width than the nozzle and is tapered at its front end so that its width is gradually reduced to that of the nozzle. The pressurizing device comprises a plate-like piezoelectric element which is disposed underneath the bottom plate within the area of the ink channel. The piezoelectric element is supported on a rigid support plate and has its top end face directly engaged with the bottom plate of the ink channel. When an electric voltage is applied to the electrodes of the piezoelectric element, the piezoelectric material expands in a vertical direction, and the elastic bottom plate is flexed inwardly of the ink channel, so that an ink droplet is expelled from the nozzle.
In a practical print head for high-speed and high-resolution printing, a plurality of ink-jet systems are integrated on a common substrate. In order to achieve objectives such as large-scale integration, a high maximum frequency of drop generation and the like, the ink-jet systems should be made as compact as possible. On the other hand, the ink jet systems should be operable with moderate voltages and must nevertheless be capable of providing a sufficient energy for creating droplets of a suitable size and accelerating them to a suitable speed so that the droplets may be deposited on the recording medium with high accuracy. It is therefore desirable to optimize the efficiency, with which the mechanical energy provided by the piezoelectric element is converted into kinetic energy of the droplet.
The total energy efficiency depends largely on the following two factors: (1) the efficiency with which the mechanical energy of the piezoelectric element is converted into energy of an acoustic wave propagating in the ink liquid and (2) the efficiency with which the acoustic energy is conferred to the droplet created at the nozzle.
The first factor is determined by the ratio between the thickness of the piezoelectric element and the depth of the ink channel. Ideally, this ratio should not be much smaller than the ratio between the elastic modules of the piezoelectric material and the ink liquid. Since the piezoelectric material generally has a comparatively large elastic module and, on the other hand, the thickness of this element is limited by practical constraints, this factor requires a rather small depth of the ink channel.
The second factor depends on the ratio between the sectional areas of the nozzle and the ink channel. Ideally, this ratio should be so selected that an optimal "impedance match" is provided for the acoustic wave, in order to avoid energy losses by reflection of the acoustic wave. Since the cross-section of the nozzle is determined by the desired size of the droplets and the width of the ink channel should not be made too large, a comparatively large depth of the ink channel would be desirable in view of this factor.
Thus, when the depth of the ink channel is determined, a compromise between the two above-mentioned factors must be made, with the result that the total energy efficiency remains rather poor.
IBM Technical Disclosure Bulletin Vol. 26, No. 10B, March 1984, discloses a different type of ink-jet system in which the ink channel is defined in the interior of a tubular piezoelectric element. The outer circumferential surface of the tubular piezoelectric element is surrounded by a plurality of discrete annular conducting bands which serve as energizing electrodes, so that a plurality of piezoelectric transducers are formed which are distributed over the length of the ink channel. If the excitation of each transducer is timed properly, a pressure wave travelling towards the nozzle in the ink channel will build up its energy as it passes under each transducer.
However, an ink-jet system of this type is difficult to manufacture, and it is particularly difficult to integrate a plurality of ink-jet systems of this type into a multiple-nozzle printhead for high-speed and high-resolution printing. In addition, since the plurality of conducting bands of each piezoelectric element in each individual ink-jet system must be energized separately, a complicated control logic is required, and the wiring system needed for applying the appropriate voltages to the individual conducting bands becomes very complex when the number of nozzles in the integrated printhead is increased.